Telecommunications equipment is typically designed for indoor temperature controlled environments, but is often deployed in outdoor locations. When telecommunications equipment is deployed in outdoor locations, a cabinet is typically used to provide environmental protection and control thermal conditions within the design limits of the telecommunications equipment.
There are essentially two main approaches to housing telecommunications equipment in outdoor cabinets--sealed cabinets and ventilated cabinets. Ventilated cabinets use natural or forced convection to draw ambient air through the cabinet to cool the equipment inside. Natural convection is only feasible for low density heat dissipation. At moderate and high power densities, forced ventilation is required.
Conventional cabinet ventilation systems use one or more fans to draw enough air into the cabinet to maintain the temperature below maximum equipment limits and to return the heated air to the outside environment. At low temperatures, the fans are turned off by a thermostat. In extremely cold climates, an electric heater is used to maintain the cabinet temperature within minimum equipment temperatures.
A drawback to the use of ventilated cabinets is that the control of the fans and heater is normally limited to on/off operation. This method of operation can lead to abrupt changes in temperature and create hot spots in the cabinet. In some cases, the equipment specifications limit the allowed rate of change in cooling air temperature to between 0.5 and 1.0.degree. C. per minute. Ordinarily, this rate of change cannot be guaranteed with conventional ventilated cabinets.
Sealed cabinets provide an alternative to ventilated cabinets for housing telecommunications equipment in outdoor locations. Sealed cabinets provide maximum protection from airborne contaminants, but require special attention to the task of removing heat dissipated by the equipment. A variety of methods are used for removing the heat dissipated by the telecommunications equipment. At moderate power densities, circulation fans, air-to-air heat exchangers, or heat pipes may be used. At high power densities, or for installations where the ambient temperature may exceed the equipment limit, an air-conditioner may be required. In extremely cold weather, electric heaters are often required to maintain the minimum equipment temperature. All of these methods for thermal conditioning have certain disadvantages or limitations.
Heat exchangers and heat pipes require a relatively large difference between the ambient temperature and the equipment cooling air temperature in order to properly transfer heat, thereby restricting the use of such an approach to cooler ambient environments. The use of air-conditioners to control temperature within sealed cabinets also presents difficulties. The heat load from the equipment in some sealed cabinets is such that the air-conditioner is required to run even at low ambient conditions, sometimes as low as 0.degree. F. Some units use variable speed-condenser blowers and/or a flooded condenser design to reduce the heat loss through the condenser and allow the air-conditioner to run at very low temperatures. Other units use a low-ambient vent to cool the cabinet with outside air when the temperature is too low to run the air conditioner. These solutions increase the cost and size of the cooling system. In addition, for small cabinets designed for locations with severe space limitations, using an air conditioner is a significant size penalty. Air conditioners are also a source of noise which may preclude their use in noise sensitive environments.